The present invention relates to metal/fiber composite materials, and in particular, to titanium alloy matrix composites.
Pure titanium is relatively soft, weak and extremely ductile. Through additions of other elements, the base metal is converted to an engineering material having unique characteristics, including high strength and stiffness, corrosion resistance and usable ductility, coupled with low density.
Titanium is allotropic. Up to 785.degree. C., titanium atoms arrange themselves in a hexagonal close-packed crystal array called alpha phase. When titanium is heated above the transition temperature (beta transus) of 785.degree. C., the atoms rearrange into a body-centered cubic structure called beta phase. The addition of other elements to a titanium base will favor one or the other of the alpha or beta forms.
Titanium alloys are classified into three major groups depending on the phases present: alpha, beta, or a combination of the two, alpha-beta. The table below lists common titanium alloy additions. The elements which favor (stabilize) the alpha phase are termed alpha stabilizers, those which favor the beta phase are termed beta stabilizers, and those which do not show a preference for either phase, but promote one or more desirable properties are termed neutral. The alpha stabilizers raise the beta transus temperature, i.e., the temperature at which the atoms rearrange from the alpha form to the beta form, and beta stabilizers lower the beta transus temperature.
The so-called beta titanium alloys are, in general, metastable. That is, within a certain range of beta stabilizer content, the all-beta matrix can be decomposed by heating the alloy to a temperature below the beta transus temperature. Such decomposition can result in allotriomorphic alpha phase or an intimate eutectoid mixture of alpha and a compound. The beta stabilizers which exhibit the former type of reaction are called beta isomorphous stabilizers while those which provide the latter reaction are called beta eutectoid.
______________________________________ Titanium Alloy Additions Alpha Beta Stabilizers Stabilizers Isomorphous Eutectoid Neutral ______________________________________ Al Mo Cr Zr O V Mn Sn N Ta Fe C Nb Si Co Ni Cu H ______________________________________
The metastable beta titanium alloys may be divided into two major groups, the rich metastable beta alloys and the lean metastable alloys. Broadly, the division of metastable beta titanium alloys is made as a result of processing and heat treatment practices: Lean metastable beta alloys retain the beta phase at room temperature only after relatively rapid cooling through the beta transus, such as by water quenching, while rich metastable beta alloys retain the beta phase at room temperature even after relatively slow cooling, such as air cooling.
The metastable beta titanium alloys may also be classified as lean or rich according to their valence electron density (VED). This value is obtained by multiplying the atomic percent of each element in the alloy by the number of its valence electrons, i.e., the number of elecrons available for combining with other atoms to form molecules or compounds, then dividing the sum of the products by 100. The alloys having a VED equal to or greater than about 4.135 may be classified as rich metastable beta alloys and those with a VED below about 4.135 may be classified as lean.
Another, perhaps more convenient method for classifying the metastable beta titanium alloys is to compare the weight percents of the beta stabilizers. In general, the metastable beta titanium alloys which contain less than about 14 weight percent total beta stabilizers may be classified as lean alloys while those which contain about 14 weight percent or more total beta stabilizers may be classified as rich.
Examples of metastable beta titanium alloys are given in the following table:
______________________________________ Total Beta Stabilizers Composition Class. VED (wt %) ______________________________________ Ti--30Mo Rich 4.352 30 Ti--13V--11Cr--3Al Rich 4.271 24 Ti--3Al--8V--6Cr--4Mo--4Zr Rich 4.176 18 Ti--15V--3Cr--3Al--3Sn Rich 4.144 18 Ti--15V Rich 4.142 15 Ti--11.5Mo--6Zr--4.5Sn Lean 4.129 11.5 Ti--10V--2Fe--3Al Lean 4.108 12 Ti--10Mo Lean 4.105 10 Ti--6.3Cr Lean 4.104 6.3 ______________________________________
In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments.
Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistent.
These titanium composites are fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic conditions, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. At the same time a reaction ocurs at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The compounds formed in the reaction zone may include TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. Titanium matrix composites have not reached their full potential, at least in part because of problems associated with instabilities of the fiber-matrix interface. The reaction zone surrounding a filament introduces new sites for crack initiation and propagation within the composite, which operates in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties are influenced by the reaction zone, that, in general, these properties are degraded in proportion to the thickness of the reaction zone.
The lean metastable beta titanium alloys need to be solution treated below the beta transus temperature for good combination of strength and ductility. Due to the coarse grain size in foil produced by current rolled foil methods, a relatively high temperature is required for compacting rolled foil (typically above the beta transus temperature), which leads to further beta grain coarsening, which in turn degrades both compactability and post compaction matrix properties.
It is, therefore, an object of the present invention to provide improved titanium composites.
It is another object of this invention to provide an improved method for fabricating titanium composites.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention and the appended claims.